An embodiment of the present invention is directed to a radiation emitter and in particular to an emitter of X-ray radiation. An embodiment of the present invention is also directed to a radiology apparatus that may include a radiation emitter and in particular an apparatus that can be used in medical imaging for example, for imaging an object. More particularly, an embodiment of the invention is directed to an apparatus for mammography. The embodiments of the present invention, however, can be applied to any other field in which radiography or a radiological examination is undertaken, to include by way of example, CT, vascular, Rad-R&F.
A conventional radiology apparatus comprises means for providing emitted radiation, such as an X-ray tube emitting X-rays. The emitted radiation is directed toward a body to be examined. The body may be any object for which a generally non-destructive investigation by imaging of the internal structure of the object is desired, such as a casting formed by metal and non-metal materials. In medical imaging, the body to be examined may be a patient's body. On another side of this body, relative to the tube, there is a detector. In practice, the detector is typically a radiographic film or an electronic detector, for example of the type with a radiology image intensifier screen.
When applied to medical imaging, depending on the nature of the lesions or structures to be revealed, there are known ways of choosing the hardness of the X-rays emitted as well as, for reasons of detection threshold, the emission power. The rays thus emitted cross the body and excite the detector in revealing the attenuation that they undergo at certain places. The problems that disturb this type of detection are of different kinds.
In particular, there are known problems of scattering in which certain parts of the body stop the emitted X-rays, without absorbing them, and become the site of Compton scattering. The rays that result from this scattering themselves also go through the rest of the body and excite the detector but, unfortunately, they do so at a place which is not straight ahead of the structure which is the site of their emission. The image is then falsified. To prevent this type of defect, there are known ways of collimating the rays by means of a grid screen which, on the whole, lets through only X-rays having a planned orientation (generally a perpendicular orientation) relative to the detector. However, to prevent traces of the grid screen from being seen in the revealed image, there are known ways of shifting the grid screen during examination. Consequently, the traces of this grid screen are distributed in the image to the point of becoming invisible.
Mammography presents a problem that is more difficult than perhaps in other radiological imaging. In mammography (but can be seen in other fields) the difficulty is related to the low level of differentiation of absorption between the healthy tissues of a breast and tissues affected by lesions, especially instances of microcalcification. To resolve this problem, X-ray emission filters that confine the spectrum of the emitted X-rays to the narrowest possible spectrum are used. The spectrum is at a value of hardness such that the rays emitted are highly absorbed by the healthy structures and less absorbed by the unhealthy structures (or vice versa) so that the contrast of the image is increased.
The production of the X-rays is obtained by the projection, at very high speeds, of electrons emitted by a cathode on an anode of the tube. Despite the choice of targets of the anode made out of appropriate materials, the spectrum of emission of the X-rays has an excessively great bandwidth. It may happen in this case that the structure to be revealed, which would have blocked rays at a given frequency, lets through rays at another frequency and the reverse would happen for other structures. This results in a loss of contrast. This is why it is desirable to filter the emission produced so as to confine the spectrum to a narrow band.
The means for filtering comprises a plate, which is generally a metal plate, interposed between the X-ray tube and the body. Thus, the interposing of a plate made of molybdenum, rhodium, aluminium, copper, gold or silver may, as the case may be, enable the choice of the range of X-rays to be used.
However, the interposing of the plate itself poses a problem. This problem is related to defects arising from the manner of manufacture of the plate, which is often obtained by rolling. The defects, which are visible in very precise conditions of examination, especially with a homogeneous interposed body (during the calibration) and with x-radiation confined in a narrow spectrum, take the form of spots or lines. In the latter case especially, the defects often have a pronounced direction that is of the axes of the rollers. Defects of this type are especially present in rhodium type filters.
The variations of attenuation of the plate may also have several other possible reasons such as the local liberation of material from the surface, local variations in thickness or variations in density.
A way to reveal these defects is a technique of contrast expansion, performed during tests of homogeneity, in a narrow window of brightness of the detected signal. The filter plates generally have a thickness of 25 to 30 micrometers.
In X-ray mammography imaging it is sought to obtain very small differences in contrast created by clinical signs such as microcalcification or masses. These biological tissues have a very low difference in coefficient of radiological transmission with the tissues whose place they take. Consequently, the spatial homogeneity of the filter must be very high. For example, it has been recognized that a local variation in attenuation, in the range of 2.5% around the nominal value of 25 to 30 micrometers, makes the images unusable. Ultimately, improved homogeneity must be acquired throughout the surface of the filter plates.